& Diol Deoxydehydration DFT Study of the Molybdenum-Catalyzed Deoxydehydration of Vicinal Diols Daniel Lupp, Niels Johan Christensen, Johannes R. Dethlefsen, and Peter Fristrup* [a] Abstract: The mechanism of the molybdenum-catalyzed de- oxydehydration (DODH) of vicinal diols has been investigat- ed using density functional theory. The proposed catalytic cycle involves condensation of the diol with an Mo VI oxo complex, oxidative cleavage of the diol resulting in an Mo IV complex, and extrusion of the alkene. We have compared the proposed pathway with several alternatives, and the re- sults have been corroborated by comparison with the mo- lybdenum-catalyzed sulfoxide reduction recently published by Sanz et al. and with experimental observations for the DODH itself. Improved understanding of the mechanism should expedite future optimization of molybdenum-cata- lyzed biomass transformations. Introduction Turning our society towards a future that does not rely on the rapid consumption of fossil reserves is one of the great chal- lenges of our time. [1] The production of fine and bulk chemicals from renewable sources is an important part of this chal- lenge. [2] One of the issues associated with biomass-derived feedstocks, such as carbohydrates, is that their oxygen content is too high for use in today’s chemical industry. [3] As a conse- quence, chemical reactions capable of reducing the oxygen content are in demand. One such reaction is the molybdenum-catalyzed deoxydehy- dration (DODH) of vicinal diols, which has recently been pub- lished. [4, 5] Its development was inspired by methyltrioxorheni- um (MTO)-catalyzed DODH, [6] which has been thoroughly stud- ied in recent years and has been expanded to employ a wide range of reductants, such as PPh 3 ,H 2 , sodium sulfite, benzyl al- cohol, and secondary alcohols, most notably the environmen- tally friendly 3-octanol. [6a] However, the high cost and low avail- ability of rhenium is a major concern for large-scale implemen- tation of the protocol. The availability and average cost of all elements have recent- ly been surveyed [7] and both factors show a clear economic and geopolitical advantage in choosing molybdenum over rhe- nium. Currently, the cost of molybdenum is about one-hun- dredth of that of rhenium. Given this large difference in price, molybdenum catalysis could prove to be more economically viable even if the protocol involving rhenium catalysis is slight- ly more efficient by, for example, requiring lower catalyst con- centration, [6o] lower reaction temperature, and so on. To support further development and refinement of molybde- num-catalyzed DODH, [4] we set out to determine a plausible mechanism for the transformation shown in Scheme 1. It has been shown that 1-hexene can be obtained in 90 % yield using half of the starting material as reductant, with pen- tanal and formaldehyde as by-products. [4] Implied oxidative di- olate cleavage has previously been shown for pinacol and glyc- erol in the reduction of sulfoxides. [8, 9] Other oxomolybdenum complexes have been used for epoxidations [10] and reductions of sulfoxides and alkenes. [11] Molybdenum-catalyzed reactions that have previously been studied by computational methods include oxidations, [12] hydrosilylations, [13] and reductions using either hydrogen [11h] or silanes. [14] In line with earlier work, [15–17] we have carried out a computa- tional study using density functional theory (DFT) to clarify the mechanism of the molybdenum-catalyzed DODH. In this arti- cle, we describe the suggested mechanism, which involves for- mation of the Mo VI bisdiolate complex, diolate cleavage, and alkene extrusion. We have examined alternative pathways, such as diolate cleavage from a dinuclear molybdenum com- plex, alkene extrusion from both mono- and bisdiolate com- plexes, as well as unmediated proton transfers in the conden- sation of molybdenum with 1,2-propanediol. The overall mech- anism is supported by its compatibility with previously pub- lished results relating to the oxidative cleavage of pinacol [8] and experimental details of the molybdenum-catalyzed DODH itself. [4] Scheme 1. Molybdenum-catalyzed deoxydehydration of 1,2-hexanediol to 1- hexene, pentanal, and formaldehyde. [a] D. Lupp, Dr. N. J. Christensen, Dr. J. R. Dethlefsen, Dr. P. Fristrup Department of Chemistry, Technical University of Denmark Kemitorvet, building 207, DK-2800 Kgs. Lyngby (Denmark) E-mail : pf@kemi.dtu.dk Supporting information for this article is available on the WWW under http ://dx.doi.org/10.1002/chem.201405473. Chem. Eur. J. 2015, 21, 3435 – 3442  2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 3435 Full Paper DOI: 10.1002/chem.201405473